Nitriding process



N 2, 1948. A. 1.. FEILD 2,452,915

I NITRIDING PROCESS Filed June 27, 1947 ALEXANDER L. FEI

Patented Nov. 2, 1948 a sama m'rammo Princess AlexanderiL. *Feild; Baltimore, :M'IL, assign'or to- Armcozsteel fiorporation, a corpora'tion of ohio Application June 27, 1947,."SerialN0; 7573126 4. Claims." (Cl. ins-@1626) 1 This application is a continuation-in part of mycopendingapplication, Serial-No.-550644;filed- August 22, 1944, now abandoned, andentitled Nitriding process andtheinvention relates to a hardening treatment for alloy steel, and more particularly to a method of nitriding stainless steel.

An object of my inventionis the-provision of a methodof nitriding'alloy steel, especiall'yst'ainlesssteel, which as a procedure is characterized by directness and simplicity; and which ensures the obtainment of uniformly nitrided steel-surfaces.

A further object is the provision'of athoroug'hly practical and highly effective methodof" rapidly and efficiently case-hardening'stainlesssteel in 'a nitrogenous atmosphere and'at the sametime' re moving "oxide film from the metal directly as apart of the nitriding procedure.

A still further object of my invention 'isthe provision of durable, wear-resistant case hardened stainless steel products which individually possess high surface hardness, which hardness moreover is uniform along the surface."

Other objects of my invention in part will be obvious and in part willbe pointed 'outhereim after.

The invention accordingly consists inthe several operational steps, and'in'the relation'o'f each of the same to one or more of thepthers as described herein, the scope of the application of which is indicated in-the following claims.

In the single figure of the accomp'anying'drawing there is schematically shown apparatus illustratively employed in the practice of my'invention.

As conducive to a clearer understanding of certain features of my invention it maybe noted at this point that stainless steel is defined as a low-carbon alloy metal which comprises from about 10% to approximately 35% 'chromium,with or without nickel, and with or without supplemental .additions of manganese; silicon, cobalt, copper, molybdenumftungsten, vanadium, colum bium, titanium, sulphur, and the like, for specialpurposes, and a balance which is substantially all iron.

There are methods of nitriding stairilesssteel" uses previously "established fora softer quality" of corrosion-resistant surface.

'An' impediment tothe formation of nitrided layers in stainless steelari-ses 'due'toa characteristic quality of surface oxide film which has a' 't'end'ency to insulate the metal fromtheeffects of conventional nitriding treatment. It ha's been' usual practice to case-"harden the stainless steels bynitriding heat-treatment as in a -current of ammonia gas of commercial-grade. To ensure satisfactory formation of nitrides' in tliis practice; however, it isnecessary first to 'cleanup the metal as by sand blasting to remove the oxide; film; Unless the initial cleaning-procedureis-followed with reasonable =promptness by the n'itriding *op-, eration and oxide film forms anew onthe metall At times the very shape or size of'a' stainless steel object makesit diflicult'or impossible to sub ject the metal with satisfaction to the essential cleaning operation. Where sand lilasting is employed; a certain "roughness which'assuch is frequently undesirable, is impartedto the metal. Thisrernains to spoilappearan'ce-or even limit the'utilityo'f the finished hardenewpro'ducti The sand-blasting or like treatment when broughtto bear upon small ordelicatepbjects, *suchas instrument dials and pointers, often results in fde formation and distortion.

"Even with this cleaning discoloration dining the'nitriding operation occasionally is encoun tere'd, this I attribute not to the fact that previous'nitriding gases are inadequately dried; but

fih'at s'ome residual oxygen-must be present "with the ammonia which combines with hydrogenssubsequently available to form water" Wh'en"the gas.

passes over thehot steel 'beingnitridedl This water causes-discoloration in the prolonged heating;

Afurt'her and quiteimportant objection to conventional nitriding procedures resides .in the frequent -'obtainment of nitrided surfaoeswl'iich "lack uniformity of hardness and'vvear-resistano'er This non uniformity apparently is due to 'inefii'ci'ent'removal of passive oxide film "initially or "it is due to reformationo'f "oxide "after the'cle'aning opera;- tionand prior to nitriding.

An outstanding object o'f'my invention, accordingly, is the provision of an economical and'industrially practicalprocess of" nitriding stainless steel productswherein"cleaninggand surface hardening are-achieved in a thoroughly. satisfactory:

. mannerwith'a minimum of, operationalsteps.

tice "of'my invention; Tnitrid'e stainless steel;

Referring now moraparticula'rly totheflprac more particularlystainless steelproducts as IQ! example tools, cutlery, machine parts, dental and surgical instruments, drafting equipment, valve elements including balls and seats, needles and other pointed objects, plumbing fixtures, trim, or ornamental pieces and the like by subjecting the same at nitriding temperature to a nitrogenous atmosphere which as initially introduced to the steel comprises an abundance both of ammonia gas and nascent hydrogen with or without nascent nitrogen. This atmosphere contains from about 15% to 85% ammonia by volume and the remainder hydrogen with or Without nitro gen, the hydrogen amounting to at least 11% by volume. And, in general, I prefer to first establish the ammonia-hydrogen atmosphere and then introduce the steel to be nitrided into this atmosphere. The nitriding treatment by way of this mixed atmosphere I find serves effectively to remove the oxide film from the metal and ensures the formation of a uniformly hard nitrided case which is resistant to wear and corrosion.

A nitrogenous atmosphere of the character described into which I prefer to introduce the stainless steel comprises such substantial amounts of composed ammonia gas, at least 15% by volume, and the gaseous products of cracked ammonia as to make available promptly an abundance of free hydrogen, at least about 11% by volume, in the vicinity of the steel for oxide film removal and nitriding purposes. I find that it is the cracked ammonia which eifects film removal and that it is-the composed ammonia which largely is responsible for the nitriding.

As illustrative of the practice of my invention, I'nitride a batch of stainless steel instrument pointers in a suitable electric, gas or oil fired batch-type metallurgical furnace H) (see the accompanying drawing) in the heating chamber of which is housed a nitriding box I I made as of 12% chromium-80% nickel alloy metal and having a lid or removable top Ha sealed in substantially gas-tight relation to the box sidewalls as with an asbestos gasket Ilb covered with aluminum foil. The instrument pointers are supported for treatment in the sealed nitriding box, preferably on horizontal screens I I of nickel or nickel alloy metal, and along with the box are furnace heated at a holding temperature suitable for nitriding which I find to be in the range of 1100 F. to 1400 F. To remove oxide film from the heating knife blades and for case-hardening these stainless steel products, I supply to the nitriding box a gaseous mixture in a current, which as initially introduced contains an abundance both of uncracked ammonia and the gaseous products of cracked ammonia, for example, about 70% ammonia by volume and 30% nitrogen and hydrogen taken together, the hydrogen amounting to about 22.5%.

For continuously supplying the treating gas to the nitriding box I prefer to employ a drum or tank l2 of commercial anhydrous ammonia from which gaseous ammonia is conducted as under lowpositive pressure as controlled by valve l2d through tube l2a to a cracking unit l3. A tube I 2b leads the gas from the cracking unit to a suitable preheating furnace [4. A tube l2) and associated valve lZe bleeds into tube l2b from tube l2 a controlled quantity of uncracked ammonia gas. At this point the gas is composed of a mixture .of cracked and uncracked ammonia. Within the heating furnace I find that any small amount of oxygen absorbed with the uncracked ammonia is given the opportunity of combining triding box through gas outlet tube Hi to suitable reclamation or disposal means (not shown).

A control is maintained over the gas composition and flow through the system just described as by means of pressure regulating valves I'Zd and |2e on the ammonia supply tank.

As treatment of the instrument pointers continues at nitriding temperature in the presence of the nitrogenous gas, which as initially introduced contains both cracked and uncracked ammonia in substantial amounts, this being from about 15%-85% uncracked ammonia and the remainder cracked ammonia, the oxide film is eifec tively removed from the surface of the articles and a hard nitrided case begins to form. Over an extended period of continued treatment the nitrided layer gradually increases to a useful thickness; usually a treatment period ranging from approximately 30 hours to hours gives a thickness which is satisfactory for most articles, though I do not wish to be bound by any such period.

Fo.lowing the nitriding treatment, I shut off the furnace and the source of ammonia supply,

and subsequently remove the nitrided stainless steel articles from the nitriding chamber. They are stored for future use orassembled and put into service as desired, either with or without further treatment such as polishing.

My method of nitriding stainless steel articles is successfully practiced on articles and products of widely differing quality, whether the products be of a form which is rugged or delicate. Due to the combined cleaning effect and strong nitriding action which prevail in the treating operation, the removalof surface oxide film from the metal is ensured and uniformly hardened surfaces are obtained on the products treated.

In the practice of my method it is possible to largely avoid preliminary treatments such as sand-blasting to remove passive oxide film, or other separate cleaning steps for the same purpose, and to avoid the many objections which frequently accompany the employment of the separate cleaning operations. Savings in time are directly effected. The products hardened as a result of my treatment are substantially free of soft spots at the surface, are uniformly resistant to corrosion, abrasion and wear, and uniformly retain surface hardness at high tempera-- tures. In cross-section the products have a nitrided case in which the nitrides are in a fine state of dispersion and impart an extreme hardness at the metal surface. The hardness gradually decreases in intensity inwardly from the sur-v face and at innermost points corresponds to that of the product core.

V The method of nitriding'which I provide is 'e'co: nomical and requires operational steps'which are easy to perform and conditions of operation which are simple to control in the mass production of nitrided stainless steel articles of out-1- standing quality. The equipment and materials needed are readily available and require little space for giving effective treatment.

Thus it will be seen that there is provided in this invention a method of case-hardening stainless steels in which the various objects hereinbefore noted together with many thoroughly practical advantages are successfully achieved. It will be noted that the method is particularly suited for rendering the stainless steels clean and responsive to nitriding action in the same atmosphere which is employed for nitriding, and that the resulting nitrided products are of outstanding quality. It will also be noted that the method cleans up the steels without necessary use of harsh abradents and thus is highly satisfactory for producing nitrided articles, even those which are delicate in construction, with a minimum of spoilage.

As many possible embodiments may be made of my invention and as many changes may be made in the embodiments hereinbefore set forth, it is to be understood that, all matter described or illustrated herein is to be interpreted as illustrative and not as a limitation.

I claim:

1. In a method of nitriding stainless steel, the art which includes cracking ammonia gas, mixing uncracked ammonia gas therewith to give a mixture consisting of about to 85% by volume of cracked ammonia and about 85% to 15% by volume of uncracked ammonia, heating the mixture of cracked ammonia and uncracked ammonia so as to form moisture from any available oxygen present with the ammonia, removing this moisture by a drying operation, then passing the preheated and predried mixture of gases over the stainless steel while held at nitriding temperature for some 30 to 90 hours to nitride the same.

2. In a method of nitriding stainless steel, the art which includes, mixing cracked ammonia gas with uncracked ammonia gas to form a mixture consisting by volume of about uncracked ammonia and about 30% cracked ammonia with the hydrogen content of the cracked ammonia amounting to about 22.5%, heating the mixture so as to form moisture from any available oxygen present with the ammonia, drying the gas mixture, and then passing the heated and dried mixture over the stainless steel to be nitrided for some 30 hours or more.

3. In a method of nitriding stainless steel, the art which includes heating a gas mixture consisting of about 15% to by volume uncracked ammonia and the remainder cracked ammonia, so as to form moisture from any oxygen present with the uncracked ammonia, drying such mixture, introducing to such gas mixture said stainless steel, and heating the same to nitriding temperature for a sufiicient length of time to remove passive oxide film from the metal surface and nitride the steel.

4. In a method of nitriding stainless steel, the art which includes heating a, gas mixture consisting of about 15% to 85% by volume of cracked ammonia and 85% to 15% by volume of uncracked ammonia so as to form moisture from any available oxygen present, removing this moisture by a drying operation, then passing the preheated and predried mixture of ases consisting of cracked ammonia and uncracke-d ammonia over the stainless steel while held at a temperature range sufiiciently high and for a period of time sufliciently long for nitriding the same.

ALEXANDER L. FEILD.

REFERENCES CITED UNITED STATES PATENTS Name Date Dahl Oct. 27, 1942 Number 

